Radio



- Nov. 8, 1932. o. T. FRANCIS RADIO Filed Nov. 17, 1928 In men 1'01.- 01iuer I? Fran dais,

Patented Nov. 8, 1932 UNITED STATES PATENT OFFICE RADIO Applicationfiled November 17, 1928. Serial No. 320,170.

This invention relates to amplifier circuits, and more particularly tothe means of coupling between the units thereof.

One object of this invention is to provide a new means of coupling,which I may designate as direct coupling, and which shall allow for moregain in amplification per stage than is possible with present day means.

Another object is to provide in the output circuit of a vacuum tubeamplifier a variable load which shall vary in accordance with somepredetermined. factor such as the current through the same or the gridvoltage impressed on said vacuum tube amplifier or a combination of thetwo, which shall'allow for greater utilization of the plate voltageimpressed on said circuit and which shall by limiting the currentflowing in said circuit allow for greater voltages to be impressed onthe same. than is possible by present day means. It is well known thatwith present day resistance coupled amplifiers that it is possible toutilize but fifteen percent of the B battery voltages applied to thecircuit.

lVith the circuit herein described practically the entire B batteryvoltage is utilized. \Vhere the majority of amplifiers utilize about tenmilliainperes of current per stage the circuit herein described utilizesbut one milliampere. This results not only in a saving of B batteriesbut in the much more important factor of allowing greater voltages to beimpressed on the plates of the tubes in the circuit without injury tothe same, as less energy will be dissipated in them with onemill-iampere flowing in the circuit than if ten milliamperes is fiowin gthrough them.

Another object of this invention is to provide a circuit where thedistortion shall be less than in present day circuits. It is well knownthat resistance coupled amplifiers are as free from distortion as anyamplifiers in common use. The cause of distortion in them is the factthat small coupling condensers of the order of one-tenth of a microfarador smaller must be utilized, and that large grid leaks of the order of amegohm must also be used for economy of the dynamic. variations inpotential. This causes distortion due to the impedance of the smallcoupling concause the same variation in voltage between densers to thelower frequencies, and also due to the fact that when strong signals arebeing received the grid of the tube to which this condenser is connectedbecomes negative because of the IR drop through the magohm resistance.By alleviation of the necessity for this economy of dynamic variations,capacities of much higher value can be used and impedances of lowervalue can be used as grid leaks.

Another object of this invention is to allow for economy in setconstruction. A casual observation of the circuit herein described willshow that no expensive transformers or output tubes are necessary assufiicien't amplification can be obtained without the aid of either. I

With these and other objects in view the features of the device can bestbe understood by a reference to the following description in connectionwith the drawing which shows diagrammatically an embodiment of the'invention.

The figure shows a multi-stage amplifier which illustrates one means ofusing a vacuum 7 tube in the input and output circuit of a second vacuumtube.

Referring to the figure we find the vacuum tube V1 coupled to theprevious stage of amplification through transformer T in accordance withthe ordinary means of transformer coupling in common use today. -A.1heats the filament of V1 and V4; A2 and A3 heat the. filaments of V2,and V3 respectively. B1 furnishes plate current for both of the stagesof amplification illustrated. V1 is connected in the input and theoutput circuit of V2. The input circuit of V2 may be traced as follows:Cathode of V2, anode of V1, cathode of V1. negative terminal of B1,point where arrow head makes contact with 131, grid of V2. The outputcircuit may be traced as follows: Anode-of V2, point P1, resistance R1,cathode of V3, anode of V3, positive terminal of B1, negative terminalof B1, cathode of V1, anode of V1, cathode of V2. Since the anode andcathode of V1 are in the cathode to control electrode path of V2, avariation in voltage between the anode and cathode of V1 will thecathode and control electrode of V2. V3 is connected in theoutputcircuits of V1 and V2, as a vacuum tube choke. This tube, V3, has a veryhigh impedance to the effective value of the alternating component ofthe plate current of vacuum tubes V1 and V2. I

- V4 is coupled to this stage thru condenser C1 by vacuum tube choke V3.In the output circuit of V4 is connected translating .de vice S.

The principles involved in this circuit'can best be brought out by ananalytical-procedure similar to that used by Morecroft Principls Y ofRadio Communication page 944. E91, E92, E93, E94, equals the effectivevalue/of alternating voltage impressed upon the grids, of V1, V2, V3,V4, respectively. I

Epl, Ep2, E723 equals the effective value of alternating voltage betweenthe anode and cathode of V1, V2, V3, respectively.

1;) equals theefi'ective value of alternating component of plate currentof V1, and V2.

1. equals the amplification constant (that is the mu) of each and everytube used in the diagram.

R171, R112, Rp3, equals the a. c. plate filament resistance of V1, V2,V3, respectively.

R1 equals the value of R1 in ohms.

RL equals the a. c. resistance of the load which in'the figure is RlplusR123.

The following equations may then be written for the figure.

Rp 1 RF+'R'L' E92 equals E'pl and since BL is very large,

Efpl approaches its maximum possible value 0 E 91 and we may substitute,fiEgl for Egl Egl) RL Rpl R122 RL Since BL is large Rpl plus R172 canbe neglected and as Egl approaches its maximum of mu plus mu squared.

In the above calculations the resistance ofthe load RL has been assumedto be so large that resistances Rpl, Rp2 can be neglected. In order forthis assumption to be justified the grids of vacuum tube generators V1,V2, must not be allowed toassume an excessive negative potential orR121, R722 will become so large that the efficiency of the circuitillustrated will be very greatly lowered as can be easily seen byreference to the above equations. The grid of V2 has therefore beenconnected to B1 at such a point as to tend to place a positive bias onthe grid of V2 in order to nearly neutralize the negative bias impressedthereon by the direct current IR drop across V1, and thereby keep thealternating current resistance of vacuum tube generator, V2, low.

' It will be understood that the filaments of V2 and V3 may be heatedeither by a separate source of A battery or by the use of tubes whichhave a separate heating element.

It is apparent that while vacuum tubes are shown as the coupling unitthat'any resistance which has the characteristic of increasing its valueas a current through same increases, such as a tungsten lamp ofsufiiciently ".small filament, could be used as the variable impedancecoupling unit.

It is further apparent that the general principles herein disclosed maybe embodied in many other organizations widely diiferent than thoseillustrated without departing from the spirit of the invention asdefined in the following claims.

It will be understood that where vacuum tube amplifier is referred to inthe claims it will include within its meaning thermionic 1tiubeamplifier or any equivalent tube ampli- I claim:

1. In an electric circuit, avacuum tube having an input and an outputcircuit, a

source of voltage for said output circuit,

means for supplying impulses to said input circuit, said meanscomprising a second vacuum tube having an anode, a cathode and a controlelectrode, said anode and said cathode being connected in said input andsaid output circuit and means for varying the po- 'tential of saidcontrol electrode.

2. In a stage of vacuum tube am lification a plurality of vacuum tubeseach aving an ing current voltages on said load, said alter-1 natlngcurrent voltages being of the same phase.

In testimony whereof, I afiix my signature.

OLIVER T. FRANCIS.

